The present invention relates to the field of electrophotographic copiers, and more specifically to the image development and paper registration control systems therefor.
In a typical electrophotographic copier, light directed to a document or other material being copied is reflected and focused onto a photoconductive surface. This surface has the property that it can accept and retain an electrostatic charge until it is illuminated, at which time the charge will substantially dissipate in proportion to the intensity of the light energy incident thereon. Thus when a focused and reflected image of the document, composed of myriad light components of varied brightness, is incident on the photoconductive surface, the areas of the surface illuminated by the brighter components will lose more charge than the areas illuminated by the less bright components. The result is a latent electrostatic image within the photoconductive surface, composed of an aggregation of charges. At a later point in the copying process, the latent image is developed by allowing charged toner particles, in either a dry or a liquid form, to be attracted toward the charged surface. The developed image is then transferred onto paper or another medium.
However, due to the rapid processing times associated with modern high speed copiers, often the most brightly illuminated areas of the photoconductive surface, which are receiving light from the white, or background, areas of the document, do not have sufficient time to dissipate the appropriate amount of their electrostatic charge. This problem is further complicated by the fact that the rate of discharge is not constant with time, but it slows down as the amount of charge decreases. As a result, a residual potential remains on the photoconductive surface corresponding to the background area which will during normal development attract the toner particles and produce a gray or dirty background in the final copy, instead of the desired white or clean background area. In addition, the background areas of documents printed on colored or off-white paper, which have lower reflectivities than a white background, will produce a higher background residual potential on the photoconductive surface than would the white paper, resulting in a gray background in the finished copy. To counteract these problems, it is known in the prior art to position an appropriately shaped development electrode close to the photoconductive surface at the point where the toner particles are deposited and to impress on the electrode a bias potential of the same polarity as the potential on the photoconductive surface. In a positive development system, this bias potential, if greater than the residual potential present on the photoconductive surface, will cause the toner particles to be attracted to the electrode rather than to the residually charged surface areas, and thus prevent the gray or dirty effect.
In the prior art, systems for imposing a fixed bias and systems for imposing a variable bias on the development electrode are well known. Typical variable bias systems can use a photodetector positioned so as to receive light reflected from the document being copied. Since the residual potential on the photoconductive surface is dependent on the intensity of the incident light, a measurement of the brightness of the reflected light during a particular cycle can be calibrated to provide a reliable indicator of the residual potential on the photoconductive surface after illumination by that same light.
In order for the previous biasing systems to be effective, however, it was necessary to insure that a photodetector would have an unobstructed view of an unmarked portion of the background of the document, for example, the margins. This constraint makes necessary either a sophisticated optical system to maintain precise alignment of the photodetector's field of view with the area of interest or an array of photodetectors aimed at various spots on the document, one of which was presumed likely to see an unmarked area. Either of these alternatives means extra expense.
Another critical function within the typical copier is the feeding of the material onto which will be transferred the developed photographic image. The transfer material must arrive at the photoconductive surface at the proper instant to insure that there will be registration or alignment of the complete image with the transfer material.
Therefore it is an object of the invention to be responsive to the reflectance characteristics of a document being copied so as to produce a finished copy combining good contrast and a shadow-free background, and to do so in a simple, reliable, and efficient manner, utilizing a minimum of components.
It is a further object of the invention to respond to reflected light of a predetermined intensity to initiate sheet feed to achieve proper registration with the image being transferred from the photoconductive surface.